The invention relates to a gas discharge lamp having a discharge vessel containing an ionizable gas filling and consisting of a translucent material, which is surrounded by a spatially separated translucent outer bulb, the discharge vessel having inside the outer bulb a thermally insulating, porous, translucent envelope.
Gas discharge lamps, more particularly high-pressure gas discharge lamps, have a discharge vessel of translucent and heat-resistant material, for example of quartz glass or sintered aluminum oxide. Conventional gas fillings of such lamps comprise, for example, sodium and/or mercury, as the case may be with additions of metal halides for improving the color rendition. In order to thermally insulate the discharge vessel, it is known to surround this vessel by a single or double-walled quartz glass tube U.S. Patent No. 2972693 and 3250934). In order to further reduce the heat dissipation, the space between the discharge vessel and the outer bulb has been evacuated. In many cases, especially in gas discharge lamps of low power, for example lower than 70 W, the thermal insulation of the discharge vessel by outer quartz glass tubes is not sufficient because the distance between the discharge vessel and the outer bulb is too small.
GB Patent No. 481320 discloses a gas discharge lamp of the kind mentioned in the opening paragraph, in which the envelope consists of glass wool. Such an envelope leads to a substantial scattering of the light emanating from the discharge vessel, as a result of which its focusing becomes more difficult. In order to avoid this, such an envelope must have only a comparatively loose packing, as a result of which the thermal insulation is limited, however.